feat: dockerize p4bl0t

This commit removes DATABASE_URL variable in favour of a fixed name.
The project won’t panic anymore if this variable isn’t set. This
removes the need for dotenvy.

It also adds the necessary files to dockerize the application.

Update instructions in README on how to run the project.

Add possibility to compile the project without a database available.

Closes #8

README.md

@@ -3,11 +3,24 @@
 p4bl0t is a simple logging bot for Discord written in Rust.
 
 ## Usage
-
-In order to run p4bl0t, head over to your [developer
+### Preparation
+In order to run p4bl0t, you will need a Discord token with which your
+bot will authenticate. Head over to your [developer
 portal](https://discord.com/developers) on Discord’s website, and
-create a bot there. Then, copy the `.env.example` file to a `.env`
-file and fill in the details.
+create a bot there. You will be able to get the bot’s token there.
+
+### Docker
+The easiest way to run p4bl0t is using Docker. Copy
+`docker-compose.example.yml` to `docker-compose.yml` and modify the
+`DISCORD_TOKEN` variable.
+
+Then, you can simply run
+```sh
+docker compose up # or docker-compose on some machines
+```
+
+### Building and running it yourself
+Copy the `.env.example` file to a `.env` file and fill in the details.
 ```sh
 cp .env.example .env
 emacs .env
@@ -27,7 +40,6 @@ cargo install sqlx-cli
 
 Setup your SQLite database.
 ```sh
-export DATABASE_URL=<your-database-url> # should be the same as in the .env file
 sqlx database create
 sqlx migrate run
 ```